1. Field of the Invention
The present invention relates to a radar provided with a transmitter circuit and a receiver circuit for transmitting and receiving radio waves in alternating cycles.
2. Description of the Related Art
In a radar provided with a transmitter circuit and a receiver circuit for alternately transmitting and receiving radio waves in successive cycles, the receiver circuit includes a frequency converter (hereinafter referred to as a mixer) for modulating a very weak echo signal received, or for converting the frequency of the received signal into an intermediate frequency. To convert the received signal into a detectable frequency, the mixer mixes the received signal with an output signal of a local oscillator (or the local oscillator signal), whereby the frequency of the received signal is converted into the intermediate frequency as discussed in Japanese Patent Application Publication No. 2002-22823. Here, the output signal of the local oscillator has a frequency separated from the frequency of the received signal by as much as the intermediate frequency.
The radar provided with the transmitter circuit and the receiver circuit for alternately transmitting and receiving radio waves typically has a single antenna used for transmission and reception. For this reason, this type of radar further includes a circulator which supplies a transmit signal output from the transmitter circuit to the antenna and delivers the received signal fed from the antenna to the receiver circuit. The circulator also serves to prevent the transmit signal output from the transmitter circuit and the output signal of the local oscillator from entering neither the receiver circuit or the transmitter circuit.
However, the aforementioned conventional circulator can not completely prevent leakage of the transmit signal fed from the transmitter circuit into the receiver circuit or of the output signal of the local oscillator from the receiver circuit into the transmitter circuit. Shown in FIG. 7 is a fundamental wave mixer used in a conventional radar. Local oscillator radio frequency (LO-RF) isolation characteristics of the mixer are determined mainly by isolation achieved by a hybrid circuit, so that the level of isolation obtained with the mixer is only about 15 dB. There exist some conventionally known approaches aimed at achieving a higher level of LO-RF isolation. Examples of these approaches are to provide a low-noise amplifier in a preceding stage of a mixer, or to insert a circulator between a transmitter circuit and a receiver circuit. Even with isolation characteristics achieved by such approaches, however, there is a problem that frequency components of the output signal of the local oscillator in the vicinity of the frequency of the received signal are radiated as spurious emissions from the antenna.
In addition, load impedance for the output signal of the local oscillator operating at a frequency close to the received signal tends to change in a conventional mixer circuit when a high-power received signal is input, so that there arises a problem related to frequency pulling.